Aluminium-immersed assembly and method for aluminium production cells

ABSTRACT

A cell for the production of aluminium by the electrolysis of a molten electrolyte, in particular the electrolysis of alumina dissolved in a molten fluoride-based electrolyte such as cryolite, comprises anodes immersed in the molten electrolyte above a cell bottom whereon molten product aluminium is collected in a pool containing bodies of aluminium-resistant material. Under the anodes are corresponding grids made of side-by-side upright or included walls aluminium-resistant material whose bottom ends stand on a ceramic-coated carbon cell bottom covered by the pool molten aluminium. The bottom ends of the grid walls form a base which is large compared to the height of the walls. Each grid on the cell bottom in stable manner during the operation of the cell and is easily removable from the cell and insertable in the cell. Each grid has generally vertical through-openings to allow the molten cell content to occupy the inside of the through-openings are in communication with the molten aluminium in the pool or layer so that the molten aluminium occupies at least a part of the height of the openings. These grids reduce movements in the aluminium pool and their top parts may act as a drained cathode.

FIELD OF THE INVENTION

The invention relates to cells for the production of aluminium by theelectrolysis of a molten electrolyte, in particular the electrolysis ofalumina dissolved in a molten fluoride-based electrolyte such ascryolite, comprising anodes immersed in the molten electrolyte above acell bottom whereon molten product aluminium is collected in a pool orlayer which contains bodies of aluminium-resistant material.

BACKGROUND OF THE INVENTION

Aluminium is produced conventionally by the Hall-Heroult process, by theelectrolysis of alumina dissolved in cryolite-based molten electrolytesat temperatures up to around 950° C. A Hall-Heroult reduction celltypically has a steel shell provided with an insulating lining ofrefractory material, which in turn has a lining of carbon which contactsthe molten constituents. Conductor bars connected to the negative poleof a direct current source are embedded in the carbon cathode formingthe cell bottom floor. The cathode is usually an anthracite or graphitebased carbon lining made of prebaked cathode blocks, joined with aramming mixture or glue.

In Hall-Heroult cells, a molten aluminium pool acts as the cathodesurface. The carbon bottom lining or cathode material has a useful lifeof three to eight years, or even less under adverse conditions. Thedeterioration of the cathode bottom is due to erosion and penetration ofelectrolyte and liquid aluminium as well as penetration of sodium intothe carbon, which by chemical reaction and intercalation causesswelling, deformation and disintegration of the cathode carbon blocksand ramming mix. In addition, the penetration of sodium species andother ingredients of cryolite or air leads to the formation of toxiccompounds including cyanides.

Difficulties in operation also arise from the accumulation ofundissolved alumina sludge on the surface of the carbon cathode beneaththe aluminium pool which forms insulating regions on the cell bottom.Penetration of cryolite and aluminium through the carbon body and thedeformation of the cathode carbon blocks also cause displacement of suchcathode blocks. Due to displacement of the cathode blocks and theformation of cracks, aluminium reaches the steel cathode conductor barscausing corrosion thereof leading to deterioration of the electricalcontact, non uniformity in current distribution and an excessive ironcontent in the aluminium metal produced.

Extensive research has been carried out with Refractory Hard Metals(RHM) such as TiB₂ as cathode materials. TiB₂ and other RHM's arepractically insoluble in aluminium, have a low electrical resistance,and are wetted by aluminium. This should allow aluminium to beelectrolytically deposited directly on an RHM cathode surface, andshould avoid the necessity for a deep aluminium pool.

Because titanium diboride and similar Refractory Hard Metals arewettable by aluminium, resistant to the corrosive environment of analuminium production cell, and are good electrical conductors, numerouscell designs utilizing Refractory Hard Metal have been proposed, whichwould present many advantages, notably including the saving of energy byreducing the anode-cathode distance (ACD).

U.S. Pat. No. 3,856,650 proposed lining a carbon cell bottom with aceramic coating upon which parallel rows of tiles are placed, in themolten aluminium, and spaced apart from one another by expansion gaps ina grating-like arrangement. The purpose of this "grating" was to protectthe ceramic coating against mechanical effects due, for example, tomovements of the aluminium pool.

U.S. Pat. No. 4,243,502 described designs for aluminium-wettablecathodes some of which had a generally horizontal active surfacesupported by one or more supporting plates, usually connected to acurrent supply by an extension protruding from the top of theelectrolyte, between the anodes. Such designs were not practicable.

U.S. Pat. No. 4,410,412 described wettable cathodes made of aluminidematerials. These cathodes were supposed to be exchangeable, by holdingseveral cathode elements together in a holder of insoluble refractorymaterial. Special cell designs to make use of such aluminides were alsodescribed in U.S. Pat. No. 4,462,886. Again, such materials and designsdid not prove to be practicable.

PCT patent application W083/04271 proposed cathodic elements ofrefractory hard materials such as titanium diboride in the shape ofmushrooms having relatively large flat tops facing the anode in order tomaximize the active cathode surface. However, no adequate means could befound for connecting the mushroom stems to the cell bottoms, so thisdesign also failed.

To accommodate for fluctuations in the level of the pool of aluminium,European patent EP-B-0 082 096 proposed the use of floating cathodeelements made of titanium diboride combined with a lighter material toreduce its density, for instance graphite. These floating elements wererestrained by elements connected to the cell bottom, leading to animpractical design.

EP-A-0 103 350 proposed the use of tubular cathode elements, for exampleof titanium diboride, which rest on the cell bottom dipping in a shallowaluminium pool. The inner diameter of the elements was such as tomaintain molten aluminium up to near the tops of the tubes by capillaryaction. These individual tubes were distributed over the cell bottomwith a suitable spacing, and were to remain on the cell bottom duringuse.

U.S. Pat. No. 4,349,427 has proposed replaceable modular cathodeassemblies in the form of a table on which free shapes of refractorymaterial are packed.

To restrict movement in a "deep" cathodic pool of molten aluminium, U.S.Pat. No. 4,824,531 proposed filling the cell bottom with a packed bed ofloose pieces of refractory material. Such a design has many potentialadvantages but, because of the risk of forming a sludge by detachment ofparticles from the packed bed, the design has not found acceptance.

U.S. Pat. No. 4,443,313 sought to avoid the disadvantages of thepreviously mentioned loose packed bed by providing a monolayer ofclosely packed small ceramic shapes such as balls, tubes or honeycombtiles having uniform, small apertures that restrain the entry of sludge.

Despite extensive efforts and the potential advantages of havingsurfaces of titanium diboride at the cell cathode bottom, suchpropositions have not been commercially adopted by the aluminiumindustry.

Recently, a number of proposals have been made for the feasible,low-cost production of various composite materials containing or coatedwith titanium diboride or other refractory ceramic materials, enablingpromising applications in many of the already-proposed cell designs.

For instance, WO 93/20027 discloses forming protective refractorycoatings on a conductive substrate like carbon starting from amicropyretic reaction layer from a slurry containing reactants in acolloidal carrier. WO 93/20026 discloses protective coatings appliedfrom a colloidal slurry containing particulate reactant or non-reactantsubstances. WO 93/25731 more particularly describes the application ofpre-formed refractory borides in a colloidal carrier to carbon cellcomponents of aluminium production cells.

Such coating materials have in particular enabled substantialimprovements in the conventional cell bottom designs. However, it hasturned out that many of the heretofore proposed "new" cell designs areunsatisfactory in one or more respects, even with materials that standup in the environment.

OBJECTS OF THE INVENTION

One object of the invention is to provide a cell in which theanode-cathode distance ACD can be made small due to there being onlysmall ripples or no ripples on the surface of the aluminium pool, or dueto there being a drained cathode configuration.

Another object of the invention is to provide means which reduce oreliminate horizontal movement of the aluminium pool which would erodethe cathode and reduce current efficiency, redissolving the metal in thebath.

A further object of the invention is to provide, in the aluminium pool,bodies of a material of low resistivity which make good contact with thecathode cell bottom, permitting a low voltage drop between the cathodecell bottom and the active cathode surface even with sludge formation.

Another object of the invention is to provide means which permitoperation of the cell with a shallow aluminium pool or layer and whichprovide a better and more uniform current distribution.

Yet another object of the invention is to provide bodies for stabilizingthe aluminium pool, which bodies are mechanically strong, easy to placeon the cell bottom, remain firmly in place during operation, canwithstand the cell conditions for long periods of time withoutdisintegrating and unwantedly depositing sludge on the cell bottom, andremain mechanically strong even after long periods of service and can belifted from the cell for servicing or replacement.

A further object of the invention is to provide a cell whose operationcosts can be reduced considerably, whose aluminium inventory can be muchsmaller than in conventional cells if desired and wherein, even when thecell is operated with a deep pool of molten aluminium,magnetohydrodynamic effects are reduced.

SUMMARY OF THE INVENTION

In its main aspect, the invention provides a cell for the production ofaluminium by the electrolysis of a molten electrolyte, in particular theelectrolysis of alumina dissolved in a molten fluoride-based electrolytesuch as cryolite, comprising a plurality of anodes immersed in themolten electrolyte above a cell bottom whereon molten product aluminiumis collected in a pool or layer containing bodies made of or coated withaluminium-resistant material.

According to the invention, the anodes are associated with a number ofcorresponding bodies each formed by a grid assembly (called a grid) ofside-by-side upright or inclined walls of aluminium-resistant material,the walls of each assembly having top ends placed under the anode andbottom ends standing on the cell bottom covered by the pool or layer ofmolten aluminium. The bottom ends of the walls form a base which islarge compared to the height of the walls so that the grid assembly whenresting on its base is stable, each such grid assembly standing on thecell bottom in stable manner during operation of the cell and beingeasily removable from the cell and insertable in the cell. Each gridassembly has generally vertical through-openings dimensioned to allowthe molten cell content (electrolyte, molten aluminium and sludge, wherepresent) to pass into the through-openings and remain inside. Thesevertical through-openings are in communication with the molten aluminiumin the pool or layer so that the molten aluminium occupies at least apart of the height of these openings.

In some embodiments at least part of the walls of the grid assembly aremade of electrically conductive material. In other embodiments where thegrid assembly remains immersed, at least part of the walls of the gridassembly are made of material of high electrical resistivity. All orpart of the walls of the grid structures may be made of or coated withan aluminium wettable material in particular a refractory boride such astitanium diboride and/or may be made of or coated or impregnated with acryolite resistant material. When the walls protrude outside the moltenaluminium to form an active cathode surface they must be made ofelectrically conductive material which is cryolite resistant andaluminium-wettable or is suitably coated to provide these properties.

The walls of each grid assembly may have aluminium-wettable top partswhich protrude above the molten product aluminium, thereby formingdrained cathode surfaces facing the associated anode.

In some drained-cathode embodiments, the protruding top parts of thegrid walls forming drained cathode surfaces are inclined, facingcorresponding inclined surfaces of the anodes, thereby facilitating gasrelease and promoting uniform wear of the anodes when they are formed ofpre-baked carbon bodies, it however being understood that dimensionallystable non-carbon anodes will usually be preferred.

Alternatively, the grids may remain totally immersed in the pool ofmolten aluminium with a stabilized surface layer of molten aluminiumover the tops of the grid walls.

The cell bottom is advantageously made of carbon or a carbon-basedmaterial, having a surface layer of electrically-conductiveRHM-containing material on which the grids stand. Such a layer is ofparamount importance because it protects the underlying carbon cathodefrom sodium penetration and avoids deformations of the cell bottom whichwould make the grid unstable. Advantageously, such coating material isof the type disclosed in WO 93/25731.

The walls of the grids are usually vertical to the plane of the gridbase, but some or all of the walls can be inclined by an angle up to 30°to the vertical from the plane of the grid base, for instance inclinedup to 15° to vertical so the top of the assembly is smaller than thebase.

In one advantageous embodiment, the grids are formed by a series ofplates intersecting one another, preferably at right angles, theintersecting plates defining a series of generally vertical openingsthrough the assembly, the intersecting plates usually having end partsprotruding from the outer faces of the two outermost plates with whichthey intersect. Such intersecting plates provide a mechanically stronggrid, which can be assembled to any desired shape and size, and whoseheight is usually much less than the width and length, so when the gridis placed on the cell bottom it will remain stable.

Other grid assemblies may be formed by tubular pieces joined togetherside-by-side, in which case the tubular pieces define a series ofgenerally vertical openings through the assembly, these openings beingprovided inside the tubular pieces and possibly also between the tubularpieces. These tubular pieces may have any desired cross-sectional shapesuch as round, square, rectangular, hexagonal etc.

The grid assemblies could also be formed by profiled sections assembledside-by-side to define a series of generally vertical openings throughthe assembly.

It is also possible to make a grid from a series of plates held inspaced-apart parallel configuration by transverse securing members suchas cross-bars.

The bottom ends of the walls may be spaced above the cell bottom or haveapertures allowing passage of molten aluminium on the cell bottom withinthe lower end part of the grid assembly between the bottom parts of thewalls forming the grid assembly. In one embodiment the top parts of atleast some of the walls of the grid assembly have recesses serving asguides which receive the lower ends of anode plates suspended above thegrid assembly. In this arrangement, advantageously other walls of thegrid assembly intersect with said recessed walls, and are made ofelectrically-conductive material, these other walls protruding above themolten product aluminium. The parts of the walls which protrude abovethe molten product alumina are made of or coated with aluminium-wettablematerial.

The assemblies according to the invention, particularly grids made ofintersecting walls, are mechanically strong, easy to place on the cellbottom, and remain firmly in place during operation. They can withstandthe cell conditions for long periods of time without disintegrating, andremain mechanically strong even after long periods of service and can belifted from the cell for servicing or replacement.

The invention also encompasses use of such cells for the production ofaluminium by the electrolysis of alumina dissolved in a molten halideelectrolyte such as cryolite, where the grids serve to restrainmovements in the pool of molten aluminium, and wherein during operationthe grids (or other assemblies) are removed periodically or whennecessary for servicing or replacement, and new or serviced grids arereplaced in the cell.

Operation of the cell is advantageously in a low temperature process,with the molten halide electrolyte containing dissolved alumina at atemperature below 900° C., usually at a temperature from 680° C. to 880°C. The low temperature electrolyte may be a fluoride melt or a mixedfluoride-chloride melt. This low temperature process is operated at lowcurrent densities on account of the low alumina solubility.

However, the invention is particularly advantageous also in conventionalcell designs where the carbon blocks are assembled to form the cellbottom, preferably with the inclusion of a refractory coating on theconventional cathode surface to support the grids. Existing cells canthus be retrofitted by inserting these grids on a coated carbon bottom.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be further described by wayof example, with reference to the accompanying schematic drawings, inwhich:

FIG. 1 is a perspective view of one example of a grid according to theinvention;

FIGS. 2a and 2b are side views of plates which can be assembledtogether, or with other similar plates, or with plates of differentshapes to form a grid according to the invention;

FIG. 3 is a partial view of an electrolytic aluminium production cellwith an anode above a grid according to the invention;

FIG. 4 is another partial view of an electrolytic aluminium productioncell with an anode having a downwardly-facing sloping surfacecooperating with inclined top parts of a cathode-forming grid accordingto the invention;

FIG. 5 is a top view of a grid assembly according to the invention withpolygonal openings; and

FIG. 6 is a side view of a different grid assembly according to theinvention showing how it cooperates with anodes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a grid 10 made by assembling together a series of plates 11and 12 at right angles to form a rectangular body having rows ofside-by-side vertical through-openings 13 opening into the top andbottom surfaces. In their bottom edges, some of the plates haveapertures 14 of suitable size and shape to allow molten aluminium topenetrate inside the grid and fill the bottom part of the openings 13when the grid 10 is placed on the cell bottom of an aluminium productioncell. As shown, the plates 11 and 12 have protruding end parts 16 and 17respectively which extend beyond the outermost plates with which theyintersect.

The height of grid 10 is small compared to the size of its base formedby the bottom ends of plates 11 and 12, so that the grid is stable whenplaced on its base. As illustrated, the height is about 1/3 the widthand length of the grid and usually it will be much less, for instance1/10th or less. At the other extreme, the height will not be less thanthe shortest side dimension, usually no more than one half the shortestside dimension. These dimensions will of course be chosen as a functionof the cell configuration in which the grids are to be used.

By way of example, FIG. 1 shows six plates 11 intersecting at 90° withsix plates 12. Any suitable number of plates may be chosen. The verticalthrough-openings 13 may be of square or rectangular cross-section, orlozenge-shaped. Usually the lengths of plates 11 and 12 will be selectedso that the grid 10 corresponds at least approximately in size to ananode of the aluminium production cell below which the grid will beplaced. However, it is possible to place two or more cathode grids 10according to the invention under one anode, or a single cathode grid 10under several anodes, for example under two anodes arranged side-by-sidein a cell, or under several anodes aligned lengthwise along a cell.

Generally speaking, a cell with a given number of n anodes will have anequal number of n corresponding grids, or a number of correspondinggrids which is a whole number multiple of n (typically 2n grids orpossibly 4n grids), or a number of corresponding grids which is a wholenumber fraction of n (typically n/2 grids where n is an even number orpossibly n/4 grids where n is a multiple of 4).

The plates 11 and 12 are made of a material resisting the conditionsencountered in an aluminium production cell, in particular the materialsshould be resistant to molten aluminium and preferably also to thecryolite or other molten halide electrolyte. The outer surface at leastof the plates 11 and 12 will preferably be made of a material wettableby molten aluminium, such as titanium diboride or anotheraluminium-wettable refractory material including composite materialsbased on titanium diboride and other refractory borides. Such refractoryborides are dense materials, which means that the grid 10 has a densitysuch that it will settle on the cell bottom and remain stable on thecell bottom during normal cell operation.

If however the plates 11 and 12 are made of a less-dense material suchas a composite formed of carbon or a carbon based material coated withrefractory boride, it may be necessary to include an internal ballast inthe walls 11, 12, for instance inner steel inserts. Or it is possible tofill one or more of the openings 13 entirely or partly with a suitabledense material. Alternatively, it would be possible to provide means forholding the grids on the cell bottom, allowing the grids to be removedwhen necessary.

Examples of walls 21, 22 are shown in FIGS. 2a and 2b. Wall 21 of FIG.2a has slots 25 in one of its long edges. Apertures 14 are providedbetween the ends of slots 25 in its edge which will rest on the cellbottom. Several of these walls 21 arranged parallel to one another canbe assembled by fitting similar walls, disposed transversely, byinterengagement of their slots 25, i.e. with the transverse walls placedupside down in relation to FIG. 2a. The top edges of the transversewalls preferably do not have recesses like the apertures 14.

FIG. 2b shows a wall 22 with slots 26 in its upper edge for receivingtransverse plates which may be held above the cell bottom by a height h,thus allowing for circulation of the aluminium pool.

The grid 10 can rest directly on the cell bottom with the bottom edgesof its plates 11, 12 on the cell bottoms, in which case apertures suchas apertures 14 are provided in the bottom edges to allow the moltenaluminium to freely penetrate into the openings 13 within the grid, orit is possible for the grid 10 to be fitted with feet on which itstands, or the grid 10 may rest on beams or walls extending across thecell bottom and which allow a space for molten aluminium to penetrate inthe bottom of the grid 10.

The intersecting walls can be held together solely by a tight fit of theinterengaging slots, or they can be welded together or secured by anysuitable means. It is also possible to make each grid with intersectingwalls as a single piece.

With reference to FIG. 3, a grid 10 of the invention, made of walls 11and 12, is illustrated on a cell bottom 30 of an aluminium productioncell, shown only in part. The cell bottom 30 is for instance made ofcarbon and is coated with a refractory coating 31, for example atitanium diboride based coating as described in WO 93/20026. Suchcoating prevents sodium penetration in the carbon cell bottom 30 and,most important, prevents deformation of the cell bottom 30. Also,particularly in the areas of the cell bottom 30 outside the grids 10,such coating improves the resistance of the cell bottom to wear bymovements of sludge.

In this example, the grid 10 is immersed in the cathodic pool of moltenaluminium 32, and normally remains permanently below the surface of themolten aluminium 32 and therefore does not normally contact the moltencryolite or other molten fluoride-based electrolyte 33. Above the grid10, an anode 34 dips into the molten electrolyte 33. As shown, the grid10 may be about the same size as the facing anode 34, but it could besomewhat smaller or larger, and may be of the same or different shape inplan view.

In this embodiment, the grid 10 serves to restrain movements in the poolof molten aluminium 32. By stabilizing the pool 32, ripples on thesurface are minimized and the anode-cathode interelectrode space can bemaintained at a small and approximately constant value, using standardconsumable pre-baked carbon anodes or, preferably, using dimensionallystable anodes.

The required number of grids 10 can be installed in place on the cellbottom 30 when starting up the cell as the cell contents melt, or duringoperation while the cell contents are already molten.

The described grids 10 made of intersecting walls are mechanicallystrong, easy to place on the cell bottom 30, remain firmly in placeduring operation, and can withstand the cell conditions for long periodsof time without disintegrating. Such grids remain mechanically strongeven after long periods of service, and they can without greatdifficulty be lifted from the cell during operation for servicing orreplacement.

FIG. 4 shows another embodiment with a grid 10 having inclined topcathode-forming edges 44 which face a corresponding inclined lower face35 of anode 34. This grid 10 comprises trapezoidal plates 41 each havinga rectangular bottom part and an inclined top edge 44. Transverse walls42 may extend to height 43, just above, at the same level as, or belowthe usual level of the surface of aluminium pool 32.

The angle of inclination of the anode face 35, and the cathode-formingedge 44 of grid 10, is usually from about 3° to about 15° fromhorizontal in order to ensure an effective removal of theanodically-generated gases, as indicated by the arrows, thereby avoiding"bubble effects" on the lower anode face, especially when the anodes 34are prebaked carbon anodes.

In this embodiment, the inclined top parts 44 of the walls 41 of grid 10protrude above the top surface of the aluminium pool 32, in the moltenelectrolyte 33. Thus, these inclined top parts 44 of grid 10 form adrained cathode from which the product aluminium drains into the pool 32which is stabilized by being held inside the vertical through-openingsin grid 10. Movements of the aluminium pool 32 between the grids 10 isalso restrained due to the presence of these grids.

Because these top parts 44 of the grids 10 are exposed both to themolten aluminium 32 and the molten electrolyte 33, these parts aresubjected to a more aggressive environment than for embodiments wherethe grid 10 remains under the cathodic aluminium 32. Consequently, thelifetime of such cathode-forming grids is not so great. However, it isrelatively easy to monitor wear or degradation of the exposedcathode-forming top parts 44 of the grids, and remove and replace anentire grid 10 when necessary or when desired to optimize cellperformance.

FIG. 5 illustrates another type of grid assembly 10 made up of severaltubular pieces 50 connected together. The illustrated assembly is madeup of a cluster of four octagonal tubular pieces 50 joined together byfacing sides 51, leaving a central opening 52 of square section. Thefacing sides 51 can be secured together, e.g. by welding, or they couldhave interengaging shapes, or both. The bottom edges of pieces 50 haveapertures for passage of molten aluminium. This cluster can be extendedby adding on further pairs of tubular pieces in either or bothdirections to form an assembly of the desired shape and dimensions.

FIG. 6 shows another cathode grid 10 cooperating with anode plates. Thisgrid comprises intersecting vertical plates 61 and 62 which rest on acell bottom 66. The plates 61 are just over half of the height of plates62. In their lower edges the plates 62 have apertures 64, below thelevel of a molten aluminium pool 72.

Mid-way between the grid's vertical plates 62, the top edges of plates61 have recesses 65 serving as guides which receive the lower ends of aseries of anode plates 74 suspended parallel to one another by means notshown.

The upper ends of the grid's cathode plates 62 protrude above thealuminium pool 72 into a molten cryolite or other molten fluoride-basedelectrolyte 63, so that electrolysis can take place between the bottomparts of anode plates 74 and the facing top parts of cathode plates 62.These protruding upper ends of cathode plates 62 are made of or coatedwith aluminium-wettable material such as titanium diboride.

I claim:
 1. A cell for the production of aluminium by the electrolysisof a molten electrolyte, in particular the electrolysis of aluminadissolved in a fluoride-based molten halide electrolyte, comprising aplurality of anodes (34) immersed in the molten electrolyte above a cellbottom (30, 66) whereon molten product aluminium is collected in a poolor layer (32, 72) containing bodies of aluminium-resistantmaterial,characterized in that the anodes (34) are associated with anumber of corresponding bodies each formed by a grid assembly (10) ofside-by-side upright or inclined walls (11, 12; 21, 22; 41, 42; 51; 61,62) made of or coated with aluminium-resistant material, the walls ofeach grid assembly having top ends placed under the anode (34) andbottom ends standing on the cell bottom (30, 66) in the pool or layer(32, 72) of molten aluminium, the bottom ends of the walls forming abase which is much larger than the height of the walls, each gridassembly (10) standing on the cell bottom (30, 66) in stable mannerduring operation of the cell and being removable from the cell andinsertable in the cell, and each grid assembly (10) having generallyvertical through-openings (13) dimensioned to allow the molten cellcontent to occupy the inside of the through-openings, said verticalthrough-openings (13) being in communication with the molten aluminiumin the pool or layer (32, 72) so that the molten aluminium occupies atleast a part of the height of said openings (13), said grid assemblieseither remaining immersed in the pool (32) of molten aluminium with astabilized surface layer of molten aluminium over the tops of the walls(11, 12) of each assembly, or protruding above the pool or layer (32) ofmolten aluminium outside and inside said grid assemblies.
 2. The cell ofclaim 1, wherein at least part of the walls of the grid assembly (10)are made of electrically-conductive material.
 3. The cell of claim 2,wherein each grid assembly (10) has walls (41) with top parts (44)having aluminium-wettable surfaces which protrude above the pool orlayer (32) of molten product aluminium to form drained cathode surfacesfacing the associated anode.
 4. The cell of claim 3, wherein the topparts (44) of the walls (41) forming drained cathode surfaces areinclined, facing corresponding inclined surfaces (35) of the anodes. 5.The cell of claim 1, wherein at least part of the walls of the gridassembly (10) are made of material of high electrical resistivity. 6.The cell of claim 1, wherein at least part of the walls of the gridassembly (10) are made of or coated with aluminium wettable material. 7.The cell of claim 1, wherein at least part of the walls of the gridassembly (10) are made of or coated or impregnated with cryoliteresistant material.
 8. The cell of claim 1, wherein the walls of thegrid assemblies (10) are made of material which is wettable by moltenaluminium and resistant to molten cryolite and electrically conductive.9. The cell of claim 1, wherein the cell bottom (30) is made of carbonor a carbon-based material, having a surface layer (31) ofelectrically-conductive RHM-containing material on which the gridassemblies stand.
 10. The cell of claim 1, wherein the walls (11, 12) ofthe grid assemblies (10) are vertical to the plane of the assembly base.11. The cell of claim 1, wherein at least some walls of the gridassemblies (10) are inclined by an angle up to 15° to vertical from theplane of the assembly base.
 12. The cell of claim 1, wherein the gridassemblies (10) are formed by a series of plates (11, 12) intersectingone another, preferably at right angles, the intersecting platesdefining a series of generally vertical openings (13) through the gridassembly.
 13. The cell of claim 12, wherein the plates (11,12) have endparts (16, 17) protruding from the outer faces of the two outermostplates with which they intersect.
 14. The cell of claim 1, wherein thegrid assemblies (10) are formed by tubular pieces (50) joined togetherside-by-side.
 15. The cell of claim 14, wherein the tubular pieces (50)define the series of generally vertical through-openings (13) of thegrid assembly, said openings being provided inside the tubular piecesand between the tubular pieces.
 16. The cell of claim 1, wherein thegrid assemblies are formed by profiled sections assembled side-by-sideto define the series of generally vertical through-openings in the gridassembly.
 17. The cell of claim 1, wherein the grid assemblies areformed by a series of plates held in spaced-apart parallel configurationby transverse securing members.
 18. The cell according to claim 1,wherein the bottom ends of the walls (11, 12) are spaced above the cellbottom or have apertures (14) allowing passage of molten aluminium onthe cell bottom within the lower end part of the grid assembly (10)between the bottom parts of the walls forming the grid assembly.
 19. Thecell of claim 1, wherein the top parts of at least some of the walls(16) of the grid assembly (10) have recesses (65) serving as guideswhich receive the lower ends of anode plates (74) suspended above thegrid assembly.
 20. The cell of claim 19, wherein other walls (62) of thegrid assembly intersecting with said recessed walls (61), and which aremade of electrically-conductive material, protrude above the moltenproduct aluminium (72).
 21. The cell of claim 20, wherein the parts ofsaid walls (62) protruding above the molten product aluminium (72) aremade of or coated with aluminium-wettable material.
 22. A method for theproduction of aluminium by the electrolysis of a molten electrolyte, inparticular the electrolysis of alumina dissolved in a fluoride-basedmolten halide electrolyte, comprising the steps of:utilizing anelectrolysis cell comprising a plurality of anodes (34) immersed in themolten electrolyte above a cell bottom (30, 66) wherein molten productaluminium is collected in a pool or layer (32, 72) containing bodies ofaluminium-resistant material, wherein the anodes (34) are associatedwith a number of corresponding bodies each formed by a grid assembly(10) of side-by-side upright or inclined walls (11, 12; 21, 22; 41, 42;51; 6, 62) made of or coated with aluminium-resistant material, thewalls of each grid assembly having top ends placed under the anode (34)and bottom ends standing on the cell bottom (30,66) covered by the poolor layer (32, 72) of molten aluminium, the bottom ends of the wallsforming a base which is large compared to the height of the walls, eachgrid assembly (10) standing on the cell bottom (30, 66) in stable mannerduring operation of the cell and being removable from the cell andinsertable in the cell, and each grid assembly (10) having generallyvertical through-openings (13) dimensioned to allow the molten cellcontent to occupy the inside of the through-openings, said verticalthrough-openings (13) being in communication with the molten aluminiumin the pool or layer (32, 72) so that the molten aluminium occupies atleast a part of the height of said openings (13); and electrolyzing saidmolten electrolyte in said cell to produce aluminium.
 23. The method ofclaim 22, wherein, during operation, the grid assemblies (10) areremoved from the cell periodically or when necessary for servicing orreplacement, and new or serviced grid assemblies are replaced in thecell.